As illustrated in FIG. 1, a thermocouple 10 is composed of two different elements 12 and 14 and a pair of screw terminals 16. In order to accurately measure the temperature at the tip 18 of the thermocouple 10, it is also necessary to accurately measure the cold junction temperature at the screw terminals 16. A common problem is that a temperature sensor for measuring the cold junction temperature at the screw terminals 16 cannot be collocated with the screw terminals due to space constraints. Further, if the screw terminals are part of PCB electronics which are removable to facilitate replacement in case of failure, the temperature sensor for measuring the cold junction temperature can not be collocated with the screw terminals 16.
In a cold junction arrangement, where the cold junction temperature sensor is PCB-mounted on a measurement PCB, the ability of the sensor to acquire the actual cold junction temperature at the screw terminals via an edge connector is dominated by a high thermal impedance of the connector region.. Relative magnitudes of other thermal paths and the total internal power dissipation and its variability will control the size of the cold junction temperature error. The most significant of these other paths is not the PCB trace or air convection, but rather the conduction through and along the PCB itself.
As discussed above, when measuring the cold junction temperature, it is often not possible to locate the sensing element precisely at the point of interest. Assuming there is a finite thermal impedance between the point of interest and the location of the sensing element, a temperature error will result whenever there is a flow of heat between these two points. This heat flow, which is equal to a difference in temperature between the point of interest and the location of the sensing element divided by the finite thermal impedance therebetween, may be internally generated within the PCB electronics or simply be due to ambient changes in temperature resulting in a flow of heat into or out of the body of the device.
Previous solutions to the problem of differing temperatures between the location of the sensing element and the location of the point of interest focused on minimizing the impedance or minimizing the heat flow, and then accepting the residual error, which is sometimes accounted for by a fixed offset adjustment. One conventional solution is to locate the sensor as close as possible to the point of interest, however, heat generated from the electronics generally causes a significant temperature difference between the temperature at the sensor and the temperature at the screw terminals.
Other conventional solutions have attempted to compensate for this temperature difference by assuming the temperature difference to be a fixed predetermined value (after a warm-up period). However, these compensation techniques are inaccurate in non-uniform temperature applications, namely, where the heat flow between the sensing point and the screw terminals is variable. In addition, these compensation techniques have a tendency to overcompensate during start-up. In contrast, the present invention accepts the fact that there will be a temperature difference due to variable heat flow, measures the variable heat flow, and corrects for this variable heat flow, in order to eliminate the temperature error under all conditions.
In particular, the method and apparatus of the present invention utilizes two sensors, neither of which are collocated with the screw terminals, measures a temperature at each of the two sensors, ensures that the heat flow between the first and second sensors and the heat flow between the first sensor and the screw terminals are equal, and determines the temperature at the screw terminals using a weighted difference of the temperatures at the first and second sensors, wherein the weighting factor is determined by the ratio of the thermal impedance between the first and second sensors and the screw terminals. The only assumption made in using this technique is that the thermal impedance between the two sensors and the thermal impedance between the second of the two sensors and the screw terminals are reasonably stable,